Voltage regulators are devices for automatically regulating and maintaining output voltage within predetermined limits in spite of variations in quantities such as supply voltage and load current. For this purpose, known regulators roughly comprise a power element between the supply and load, and a regulating loop consisting of a differential amplifier input-connected to a stable reference voltage and to the regulator output, and output-connected to a gain stage for so driving the power element as to withstand any variation in output voltage. Known regulators frequently also present a shorting protection circuit for detecting the load current and intervening in the event this exceeds a predetermined value. The output voltage is normally picked up by the differential amplifier via a resistive divider.
Recently, a demand has arisen for what are known as "programmable-output" voltage regulators, that is, capable of providing for at least two different output values. These are particularly requested for so-called "smart cards," the standards governing which demand that the rate at which the output voltage varies during transient states falls within a predetermined range, regardless of the type of load or output voltage.
Such a condition is particularly hard to meet when the load is of a capacitive type, in which case, the regulator is called upon to supply a high current during transient states in which the output voltage increases from the steady-state to a higher value, and to absorb the load current during transient states in which the output voltage switches from the higher to a lower value, while at the same time ensuring the above condition is met.
Known voltage regulators fail to cater for the above requirements, owing to the fact that, at the beginning of the output voltage rising transient state, a short-circuit condition is produced by the output capacitor, thus resulting in undesired operation of the shorting protection circuit; whereas, during the output voltage decreasing transient state, voltage decreasing depends on the condition of the load, by virtue of the discharge current of the capacitor being absorbed by the resistive part of the load. In either case, therefore, the voltage variation caused by switching between two stable steady-state values is uncontrolled, and may assume levels that are unacceptable by present standards.